When hydrocarbon producing wells are drilled, initial hydrocarbon production is usually attained by natural drive mechanisms (water drive, solution gas, or gas cap, e.g.) which force the hydrocarbons into the producing wellbores. If a hydrocarbon reservoir lacks sufficient pore pressure (as imparted by natural drive), to allow natural pressure-driven production, artificial lift methods (pump or gas lift, e.g.) are used to produce the hydrocarbons.
As a large part of the reservoir energy may be spent during the initial (or "primary") production, it is frequently necessary to use secondary hydrocarbon production methods to produce the large quantities of hydrocarbons remaining in the reservoir. Steam-flooding is a widespread technique for recovering additional hydrocarbons and usually involves an entire oil or gas field. Steam is injected through certain injection wells selected based on a desired flood pattern, lithology, and geological deposition of the pay interval. Displaced oil is then produced into producing wells in the field.
Ability to displace oil from all the producing intervals in a hydrocarbon reservoir is limited by the lithological stratification of the reservoir. That is, there are variations in permeability which allow higher permeability zones to be swept with injected fluid first and leave a major part of the hydrocarbon saturation in lower permeability intervals in place. Continued injection of flooding fluid results in "breakthrough" at the producing wells at the high permeability intervals which renders continued injection of the flooding medium uneconomical.
Profile control has been used to prevent or correct "breakthrough" at high permeability intervals. Profile control involves using stable polymers to retard or, in some cases, block off the higher permeability intervals in a mature flood so that the flooding media is diverted to the lower permeability intervals. Field experience has indicated profile control can be used to enhance productivity from lower permeability intervals.
To impart profile control, specially developed polymers (such as copolymers and polysaccharides) are used to retard permeability of higher permeability intervals. However, permeability damage is imparted to the low permeability zone while the profile control material is being used to treat the high permeability interval(s).
One method where gels have been used for profile control is discussed in U.S. Pat. No. 4,848,464 which issued to Jennings, Jr. et al. on Jul. 18, 1989. As taught in that patent, a zone of lesser permeability was closed by injecting gel into a formation during profile control. A solidifiable gel containing a gel breaker was injected into the formation where it entered into a zone of lesser and a zone of greater permeability. Another solidifiable gel lacking a gel breaker was injected into the zone of greater permeability where it subsequently solidified. Gel contained in the zone of lesser permeability liquified thereby unblocking that zone. Afterwards, a waterflooding enhanced oil recovery method was directed into a zone of lesser permeability.
Another method where gels are used in profile control is discussed in U.S. Pat. No. 4,856,586 which issued to Phelps et al. on Aug. 15, 1989. In this method, a rehealable Xanthan biopolymer was combined with a cross-linked non-selective polyacrylamide polymer gel. The combined gel system was injected into a formation where the Xanthan biopolymer gel selectively entered a zone of lesser permeability carrying therewith said non-selected gel. Once in the formation's zone of greater permeability, the gel rehealed and formed a rigid gel with substantially better temperature stability.
Although the above-mentioned methods are satisfactory in certain applications, sometimes it is necessary to precisely place a gel within a location in a formation to obtain better profile control.
Therefore, what is needed is a profile control method where a high permeability zone can be closed to fluid flow with a great degree of accuracy while minimizing damage to a lower permeability zone.